Currently, DS-CDMA cellular systems are classified as inter-cell synchronous systems with precise inter-cell synchronization and asynchronous systems without it. For inter-cell synchronous systems, an identical long code is assigned to each base station, but with a different time offset. The initial cell search can be executed by performing timing acquisition of the long code. The search for a peripheral cell on hand-over can be carried out quickly because the mobile station can receive the offset information of the long code for the peripheral base station from the current base station. Therefore, each base station requires a precise-time synchronization apparatus, such as the global position system (GPS) and rubidium backup oscillators. However, it is difficult to deploy GPS in basements or other locations where RF signals cannot easily reach.
In asynchronous systems such as wide-band CDMA and 3GPP, each base station adopts two synchronization channels such that a mobile terminal can establish the link and will not lose the connection on hand-offs by acquiring the synchronization codes transmitted in synchronization channels. The first synchronization channel (primary synchronization channel, hereinafter PSCH) consists of an unmodulated primary synchronization code (denoted as Cpsc) with length of 256 chips transmitted once every slot. Cpsc is the same for all base stations. This code is periodically transmitted such that it is time-aligned with the slot boundary of downlinik channels. The secondary synchronization channel (hereinafter SSCH) consists of a sequence of 15 unmodulated secondary synchronization codes (Cssci,0 to Cssci,14) repeatedly transmitted in parallel with Cpsc in the PSCH. The 15 secondary synchronization codes are sequentially transmitted once every frame. Each secondary synchronization code is chosen from a set of 16 different orthogonal codes of length 256 chips. This sequence on the SSCH corresponds to one of the 64 different code groups which the base station downlinik scrambling code belongs to. The code allocation for a base station is shown in Table 1 as illustrated in FIG. 9. These 64 sequences are constructed such that their cyclic-shifts are unique. In other words, if the count of cyclic-shifting is 0 to 14, all 960 (=64*15) possible sequences generated by cyclic-shifting the 64 sequences are different from each other. Base upon this property, cell search algorithms can be developed to uniquely determine both the code group and the frame timing.
During the initial cell search for the wide-band CDMA system proposed by 3GPP, a mobile station searches for the base station to which it has a lowest path loss. It then determines the downlinik scrambling code and frame synchronization of the base station. As is well known in digital communication, a stream of framed data is transmitted and frame synchronization is the important process by which incoming frame signals of a stream of framed data are identified so that the data bits within the frame can be extracted for decoding or retransmission. This initial cell search is typically carried out in three steps:
Step 1: Slot Synchronization
During the first step of the initial cell search procedure, the mobile station searches for the base station to which it has lowest path loss via the primary synchronization code transmitted on the PSCH. This is typically done with a single matched filter matching to the primary synchronization code. Since the primary synchronization code is common to all the base stations, the power of the output signal of the matched filter should have peaks for each ray from each base station within a receivable range. The strongest peak corresponds to the most stable base station for linking. Detecting the position of the strongest peak yields the timing and the slot length that the strongest base station modulates. That is, this procedure allows the mobile station to acquire slot synchronization to the strongest base station.
Step 2: Frame Synchronization and Code-Group Identification
During the second step of the cell search procedure, the mobile station utilizes the secondary synchronization code in the SSCH to find the frame synchronization and the code group of the cell found in the first step. Since the secondary synchronization code is transmitted in parallel with the primary synchronization code, the slot timing of the secondary synchronization channel can also be found during the first step. The received signal at each time slot of the secondary synchronization channel is consequently correlated with 16 possible secondary synchronization code word symbol signals for code word symbol identification for code identification. The 15 consecutive code word symbols received and identified within one frame construct a received sequence. By sending the received sequence into a Reed-Solomon Decoder or by correlating the received sequence with the 960 possible sequences, the code group for the synchronized base station as well as the frame synchronization can be determined.
Step 3: Scrambling-Code Identification
During the last step of the cell search procedure, the mobile terminal determines the exact primary scrambling code used by the found base station. The primary scrambling code is typically identified through symbol-to-symbol correlation over the Common Pilot Channel (hereinafter CPICH) with all codes within the code group identified in the second step. After the identification of the primary scrambling code, the Primary Common Control Physical Channel (hereinafter PCCPCH) can be detected. Then the system- and cell-specific information can be read.
In summary, the main tasks of the initial cell search procedure are to (1) search for a cell with the strongest received power, (2) determine frame synchronization and code group, and (3) determine the down-link primary scrambling code.
The cell search procedure (2) is the subject of this invention. The SSCH is used to determine frame synchronization. A frame of 15 SSCH symbols forms a code word sequence taken from a codebook of 64 different code word sequences. The same code word sequence is repeated every frame in a cell. The 64 code word sequences are chosen to have distinct code phase shifts, and any phase shift of a code word sequence is different from all phase shifts of all other code word sequences. With these properties, the frame boundary can be detected by identifying the correct starting phase of the SSCH symbol sequence. In order to satisfy the above properties and maximize the minimum distance between different code word sequences, a (15,3) Comma-Free Reed-Solomon Code over GF(16) is proposed.
The standard Reed-Solomon decoder for (15,3) Comma-Free Reed-Solomon can be found in textbooks about error correcting codes and can correct up to 6 symbol errors. However, due to the frequency error, channel fading, channel noise or other reasons, the number of symbol errors may exceed 6 frequently. Therefore, the standard Reed-Solomon decoder fails to return a valid code word.
Another method is proposed by Yi-Ping Eric Wang in “IEEE Journal on Selected Areas in Communications vol. 18, no. 8 August 2000”. Wang proposed that after achieving slot synchronization, the receiver operations start with correlating the received signal of SCH with all 16 S-SCH sequences, and then accumulates SSCH correlations over Nt slots according to the 64 Reed-Solomon code word sequences used, each with 15 hypothesized frame boundaries. The total number of hypotheses is therefore 960. At the end, the hypothesis with the largest accumulated metric is chosen as the candidate for frame boundary-code group pair, which is given to next stage for scrambling code identification.
The method proposed by Wang has better performance, but it needs large amount of memory and large amount of computation work. In our invention, we provide a power- and memory-effective method by use of standard Reed-Solomon decoder combined with reliability measurement.